1. Field of the Invention
The present invention relates to bags for use in the preservation of blood cells.
2. Discussion of Prior Art
Blood used for transfusion is normally stored in refrigerators, at a temperature of 4.degree..+-.2.degree., where its safe life is no longer than six weeks. Potentially longer storage life might be obtained by freezing, but unfortunately freezing blood as obtained from a donor results in destruction of the red blood cells making the blood totally unsuitable for transfusion. Red blood cells can be stored after separation from other blood components, by centrifuging, plasma, mixing with a cryoprotectant, and then freezing. The usual cryoprotectant is glycerol. As glycerol itself is poisonous frozen cells must have all the glycerol removed therefrom before being used for transfusion. The necessary removal process, involving centrifugation and multiple washing steps, is a skilled and time-consuming task which also results in a significant loss of viable blood cells.
Another cryoprotectant is hydroxyethyl starch (HES), which is a widely used artificial plasma expander and is non-toxic. Much work has been carried out on methods of preserving red blood cells using this substance. Early efforts were unsuccessful, as it was found that the level of haemolysis (breakdown of red blood cells) in units of blood (a unit of blood being the volume of a standard donation, about 450 ml) recovered after freezing was above the safe limit. It is usually considered that a unit of blood is safe for transfusion if the level of haemolysis at thaw is no greater than 1%. A method of preserving and recovering red blood corpuscules by freezing, using HES as a cryoprotectant, wherein the level of haemolysis after recovery is within acceptable limits, has now been developed and is described in Patent Application PCT/GB90/0140.
The bags used in the method of PCT/GB90/0140 must be capable of fulfilling stringent criteria. Freezing is carried out in liquid nitrogen, and is accompanied by unavoidable changes in volume of the mixture of red blood cells and cryoprotectant. As explained in PCT/GB90/0140 the manner in which this change of volume is accommodated by control of the thickness of the freezing bag during freezing is critical. It is also important that the concentration of HES is within stringent limits, and that the process of transferring donated blood to the freezing bag is carried out in a sterile manner.
It has been suggested that the HES be stored in the freezing bag itself. However it has been found that the materials suitable for manufacturing the bag are slightly porous, allowing the concentration of HES therein to vary, the nature of the variation being dependant on the atmospheric humidity of the storage area.
It is also important that the bags, which must, once filled with blood, be stored at sub-zero temperatures of, for example -100.degree., should be of a convenient shape for storage. It must also be possible to allow the blood, after thawing to flow from the bag in a sterile manner for transfusion purposes. Bags which fulfil these criteria are described in Patent Applications WO 89/04280 and WO 91/11968. The manner of extraction from these involves penetration by a needle of a special port built into the bag. This procedure requires skill, presents the blood in a format not usually recognised by nursing staff, and also breaches the absolute microbiological integrity of the bag. A fluid storage bag having a single port, described as an outlet port, is illustrated in U.S. Pat. No. 4,256,333. The port is formed from a tube attached to the bag and a closure, and the disclosure is concerned with details of the joint between the tube and the closure.